1. Field of the Invention
The present invention relates to an ultrasound probe which includes a composite piezoelectric layer which realizes high sensitivity and wider bandwidth, an ultrasound diagnostic imaging apparatus and a manufacturing method of the ultrasound probe.
2. Description of Related Art
Ultrasound diagnostic imaging apparatuses of high image quality are desired, and making ultrasound probes realize high sensitivity and wider bandwidth is suggested as a method for realizing high image quality.
An ultrasound probe makes an piezoelectric material excite elastic vibration whose thickness corresponds to ¼) and emits ultrasound generated by the elastic vibration to a subject. With respect to the ultrasound energy emitted in the direction opposite to the direction in which a subject is positioned, since the ultrasound is reflected in the direction opposite to the direction toward a subject by an acoustic reflection layer having high acoustic impedance, which is disposed on the side opposite to the side of the subject, the ultrasound energy to be emitted is increased and high sensitivity can be realized.
Further, a composite piezoelectric layer is used as a member for generating ultrasound. A composite piezoelectric layer is formed of a piezoelectric material such as lead zirconate (PZT) and a polymer material such as resin wherein the materials are arranged alternately in the direction orthogonal to the direction toward a subject who is to be emitted and the materials are integrated. The composite piezoelectric layer is made to have low acoustic impedance by being provided with a polymer material and is made to have acoustic impedance close to that of a subject which is low comparing to the acoustic impedance of the piezoelectric material. Thereby, high sensitivity and wider bandwidth can be realized.
However, taking the above measures is not enough to realize high sensitivity and wider bandwidth. FIG. 11 shows that the thickness of an adhesion layer between the piezoelectric material and the acoustic reflection layer has relevance with the frequency band width. The horizontal axis shows frequency (MHz) and the vertical axis shows response sensitivity (Loop Gain) of piezoelectric material with respect to frequency in decibel (dB). The curve A shows the case where the thickness of the adhesion layer between the piezoelectric material alone and the acoustic reflection layer is 1.5 μm, the curve B shows the case where the above thickness is 1.0 μm and the curve C shows the case where the above thickness is 0.5 μm. As it is clear from the curves, as the thickness of the adhesion layer that attaches the piezoelectric material and the acoustic reflection layer is thinner, the frequency band at the desired response sensitivity of the piezoelectric material is wider. Therefore, in a composite piezoelectric layer in which a piezoelectric material and a polymer material are arranged alternately in the direction orthogonal to the direction toward a subject to be irradiated, the frequency band also becomes wider if the thickness of the adhesion layer which attaches the piezoelectric material parts in the composite piezoelectric, layer and the acoustic reflection layer becomes thinner. That is, making the thickness of the adhesion layer which attaches the piezoelectric material parts in the composite piezoelectric layer and the acoustic reflection layer thinner is important in order to realize even wider bandwidth.
In order to realize this, the adhesion layer between the piezoelectric material parts in the composite piezoelectric layer and the acoustic reflection layer needs to be made thin by performing the bonding by the adhesion layer after performing mirror polishing on the bonding surfaces of the composite piezoelectric layer, which is to be bonded with the acoustic layer by the adhesion layer, and smoothen the bonding surfaces before the bonding.
However, because the hardness of the piezoelectric material such as PZT which forms the piezoelectric material and the hardness of the polymer material formed of a resin differ from each other, when mirror polishing is to be performed on the bonding surface of the composite piezoelectric layer, the piezoelectric material will be polished more comparing to the polymer material. As a result, the bonding surface of the piezoelectric material and the polymer material will be bumpy and the piezoelectric material parts cannot be smoothened sufficiently causing a problem that the thickness of the adhesion layer cannot be uniformly and sufficiently thin.
In view of such problem, JP 2009-61112 discloses a technique for manufacturing a composite piezoelectric layer by cutting out the piezoelectric material to their midpoint and not all the way to the bottom and filling the polymer material in the spaces formed by cutting out the piezoelectric material. It is described that because the entire bonding surface that bonds with the acoustic reflection layer is the piezoelectric material in the composite piezoelectric layer formed by the above method, bumps due to difference in hardness between the piezoelectric material and the polymer material does not occur even if mirror polishing is performed; and thus, the adhesion layer can be thin.